Spin-dependent tunneling (SDT) effects are believed to depend upon a quantum mechanical probability of electron tunneling from one ferromagnetic (FM) electrode to another through a thin, electrically nonconductive layer, with the probability of tunneling depending upon the direction of magnetization of one electrode relative to the other. SDT effects have many potential applications in magnetic field sensing devices, such as magnetic field sensors and information storage and retrieval devices. Read transducers for magnetic heads used in disk or tape drives, which may be termed magnetoresistive (MR) sensors, and solid-state memory devices such as magnetic random access memory (MRAM), are potential commercial applications for spin tunneling effects.
SDT devices typically include two FM electrodes and an electrically insulating tunneling barrier. One of the electrodes may include a pinned ferromagnetic layer and the other may include a free ferromagnetic layer. The pinned layer typically consists of a FM layer that has its magnetic moment stabilized by a pinning structure. The pinning structure may be an antiferromagnetic (AFM) layer that adjoins the pinned layer. The magnetic stabilization can be enhanced by using a synthetic AFM structure as the pinned layer. The synthetic AFM structure includes a transition metal such as ruthenium (Ru) in a sandwich between two FM layers, in which the transition metal layer has a precisely defined thickness that is typically less than 10 Å. The magnetization direction of the pinned FM layer may be after deposition by annealing in a magnetic field. The free layer is typically a magnetically soft FM layer. The tunneling barrier may be made of a thin dielectric layer, such as Al2O3 or AlN, which has a thickness typically in a range between 0.5 nm and 2 nm.
FIG. 1 shows a top view and FIG. 2 shows a cross-sectional view of a prior art MRAM device. SDT cells 20 and 22 are connected to an electrically conductive bit line 30, and SDT cells 24 and 26 are connected to another electrically conductive bit line 33. SDT cell 24 includes a ferromagnetic (FM) free layer 35 separated from a FM pinned layer 37 by a tunnel barrier layer 39. SDT cell 26 includes a FM free layer 40 separated from a FM pinned layer 42 by a tunnel barrier layer 44. SDT cell 24 is connected to an electrically conductive read line 46 that is coupled to ground 50 by a transistor 48, which includes source region 52, drain region 54 and gate 56. Similarly, SDT cell 26 is connected to an electrically conductive read line 58 that is coupled to ground 50 by a transistor 60, which includes source region 62, drain region 54 and gate 66. Electrically conductive word line 75 controls the voltage on gate 56, and electrically conductive word line 77 controls the voltage on gate 66. Other read lines 80 and 82 exist for cells 20 and 22, respectively.
Electrically conductive digit line 70 is disposed adjacent to SDT cell 24 so that current flowing in digit line 70 and bit line 33 can change the magnetization direction of free layer 35, writing information to SDT cell 24, while transistor 48 is turned off. Similarly, electrically conductive digit line 72 is disposed adjacent to SDT cell 26 so that current flowing in digit line 72 and bit line 33 can change the magnetization direction of free layer 40, writing information to SDT cell 26, while transistor 60 is turned off.
The magnetic direction of free layer 35 relative to pinned layer 37 can act as a switch in determining whether electrons can tunnel through the barrier layer 39. When the magnetic moment of the free layer 35 is parallel to that of the pinned 37 layer electron tunneling is more likely than when the magnetic moment of the free layer is antiparallel to that of the pinned layer. This change in the amount of tunneling to an applied magnetic field may be termed magnetoresistance and can be measured as a change in current, resistance or voltage across the SDT device.
To read the information or state stored in SDT cell 24, transistor 48 is turned on and the voltage of bit line 33 indicates whether free layer 35 is parallel to or antiparallel to pinned layer 37. Even when free layer 35 and pinned layer 37 are parallel, resistance is encountered across tunnel barrier 39, and additional resistance may be present in bit line 33, read line 46 and transistor 48.
A figure of merit for a SDT device is the change in resistance divided by the resistance (AR/R) of the device in response to a change in applied magnetic field. For a MRAM device the MR is related to the voltage or logic levels that can be read with the device. Insufficient MR compared to required voltage levels could lead to errors, excessive current and/or power being usage and excessive heat generation.